1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece, and more particularly to a polishing apparatus for polishing a workpiece having a thin film formed thereon, such as a semiconductor wafer, to a flat mirror finish. The present invention also relates to a method of dressing a polishing surface, and more particularly to a method of dressing a polishing surface serving to polish a workpiece such as a semiconductor wafer by sliding contact.
2. Description of the Related Art
As semiconductor devices have become more highly integrated in recent years, circuit interconnections have become finer and distances between those circuit interconnections have become smaller. In a case of photolithography, which can form interconnections that are at most 0.5 μm wide, it is required that surfaces on which pattern images are to be focused by a stepper should be as flat as possible because a depth of focus of an optical system is relatively small. A polishing apparatus for performing chemical mechanical polishing (CMP) has been used for planarizing such a semiconductor wafer.
In a polishing apparatus for polishing and planarizing a surface of a semiconductor wafer on which a device pattern is formed, a non-woven fabric has heretofore been used as a polishing cloth attached to an upper surface of a polishing table. However, as ICs (integrated circuits) and LSIs (large scale integrated circuits) have become more highly integrated in recent years, it is required to reduce the difference in the level of a surface, to be polished, of the device pattern by polishing. In order to meet such demand for reducing a difference in level of a surface, to be polished, of the device pattern, a polishing cloth made of a hard material, e.g., a polishing cloth of foamed polyurethane, has been used in a polishing apparatus.
This type of polishing apparatus comprises, as shown in FIG. 1, a polishing table 502 having a polishing pad (polishing cloth) 500 attached thereon, and a top ring 504 for holding a workpiece W to be polished, such as a semiconductor wafer, in a manner such that a surface to be polished faces the polishing table 502. The polishing pad 500 has an upper surface serving as a polishing surface. In this polishing apparatus, a semiconductor wafer W is polished as follows. Polishing table 502 and top ring 504 are independently rotated, and, while a polishing liquid is supplied from a polishing liquid supply nozzle 506 provided above polishing table 502, the semiconductor wafer W is pressed against polishing pad 500 on polishing table 502 at a predetermined pressure by top ring 504. For example, a suspension of fine polishing particles of silica or the like in an alkali solution is used as a polishing liquid supplied from polishing liquid supply nozzle 506. Thus, the semiconductor wafer W is polished to a flat mirror finish by the combined effect of a chemical polishing effect attained by the alkali and a mechanical polishing effect attained by the polishing particles.
When a semiconductor wafer W is continuously polished in a state such that the semiconductor wafer W is brought into contact with polishing pad 500 which is rotated together with polishing table 502, polishing particles or polishing wastes are attached to polishing pad 500, resulting in a change in properties of polishing pad 500 and a deterioration in polishing performance. Therefore, if polishing pad 500 is repeatedly used for polishing semiconductor wafers W, problems such as lowered polishing rate and uneven polishing are caused. In order to overcome such problems, conditioning called dressing is performed before, after or during polishing of a semiconductor wafer to regenerate the polishing pad.
In order to perform a dressing process of a polishing pad, a dresser 508 is provided in a polishing apparatus, as shown in FIG. 1. Polishing pad 500 may be dressed by dresser 508 at the time, for example, of replacement of a semiconductor wafer W to be polished. Specifically, while a dressing element attached to a lower surface of dresser 508 is pressed against polishing pad 500 on polishing table 502, polishing table 502 and dresser 508 are independently rotated to remove polishing particles and polishing wastes attached to the polishing surface and to flatten and dress the entire polishing surface. Thus, the polishing surface is regenerated by a dressing process.
In order to enhance a capability of holding a polishing liquid on a surface of polishing pad 500, concavities such as grooves or fine holes may be formed in the surface of polishing pad 500. When a polishing process is repeatedly performed, these concavities in polishing pad 500 may be clogged with polishing particles or polishing wastes. Further, in a dressing process, the concavities in polishing pad 500 may be clogged with dressing wastes produced by dressing polishing pad 500. If the concavities formed in polishing pad 500 are clogged with such foreign matter, then a capability of holding a polishing liquid on a surface of polishing pad 500 is lowered to thereby deteriorate polishing performance. Specifically, after an identical polishing pad is repeatedly used to polish a number of semiconductor wafers, a polishing rate is relatively lowered at a central portion of a surface of a semiconductor wafer due to the foreign matter clogging the concavities. Therefore, a semiconductor wafer cannot be polished as expected.